An append-only data storage system is described that stores sets of data blocks in extents that are located in storage devices. During operation of the system, upon receiving a request to copy an extent from a source storage device to a destination storage device, the system creates a scratch extent on the destination storage device, and associates the scratch extent with a private identifier, whereby the scratch extent can only be accessed through the private identifier. The system uses the private identifier to perform a copying operation that copies the extent from the source storage device to the scratch extent on the destination storage device. After the copying operation is complete and the scratch extent is closed, the system associates the scratch extent with a public identifier, whereby the copy of the extent on the destination storage device becomes publically accessible to other entities in the data storage system.